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The kidneys have hemopoietic, reticuloendothelial, endocrine, and excretory functions. They are located retrocoelomically and vary from parallel, paired structures to a fused single structure. Freshwater fish have larger kidneys (Helfman et al. 2009). In bony fish, kidneys are divided into a cranial (head) kidney and a caudal (excretory) kidney (Figure A1.15). Lymphoid tissue predominates in the cranial kidney; thyroid tissue may also be found (Stoskopf 1993). Excretory function is primarily in the caudal kidney. Ureters move urine from the collecting ducts to either urinary papilla or a urinary bladder where water resorption occurs. This bladder is not homologous to the mammalian bladder as it develops from the distal ureter and is not mesodermal in origin (Stoskopf 1993).

Internal body fluid composition and maintenance is complex and environmental circumstances play a huge role in how fish manage osmoregulation. Fluid exchange occurs at the gill, gastrointestinal tract, and kidney. In freshwater teleosts, osmoregulation is marked by a large influx of water from the environment and production of a large volume of dilute urine (Table A1.2). Sodium is taken up by the gills in exchange for protons in a process that is pH‐dependent. Marine teleosts do drink water but do not absorb water through the gastrointestinal tract; instead sodium and chloride tend to diffuse into the fish causing salt overload. The salt is mostly excreted by the gills; the kidneys are more involved in excretion of magnesium and sulfate. Marine teleosts have fewer glomeruli than freshwater teleosts and some marine teleosts are aglomerular, e.g. some seahorses (Syngnathidae), toadfish (Opsanus spp.), and goosefish (Lophiidae). Marine teleosts are also missing the distal segments of the nephron, including the loop of Henle. They cannot concentrate their urine above their blood osmolality and are prone to dehydration. In euryhaline species, the urinary bladder changes permeability based on environmental osmolality and regulates sodium and chloride removal. Some fish migrate between freshwater and marine environments. Fish that move from freshwater to saltwater to spawn are catadromous, e.g. true eels (Anguillidae). Fish that move from saltwater to freshwater to spawn are anadromous, e.g. salmonids (Salmonidae), sturgeon (Acipenseridae). For a full review, readers are directed to Hoar and Randall (1969), Hoar et al. (1983), Stoskopf (1993), and Evans et al. (2004).

Figure A1.15 Kidneys in a deacon rockfish (Sebastes diaconus): cranial kidney (Cr Kd), caudal kidney (Ca Kd), opened swim bladder (SB) showing the rete, and liver (L).

Source: Image courtesy of Catherine Hadfield, Seattle Aquarium.

Table A1.2 Fluid and electrolyte balance in freshwater and marine bony fish.

Freshwater bony fish	Marine bony fish
Hypertonic compared to the environment	Hypotonic compared to the environment
Do not drink water	Drink water
Active excretory process	Passive excretory process
Excrete large volumes of dilute urine	Excrete small volumes of urine
Ions maintained by gill and gastrointestinal uptake	Gills excrete Na and Cl. Urine is similar to plasma
	Easily become dehydrated

Urine collection is possible but not easy. In most species, urine flow is low and continuous, limiting opportunities for collection via traditional veterinary techniques (e.g. percutaneous or catheter passage). Catheter implantation or surgically fitted collection devices are possible, but the diagnostic value of these samples is not understood (Stoskopf 1993).